1. Field of the Invention
The present invention relates to an apparatus for polishing a face of a slab member such as a silicon wafer.
2. Prior Art
Generally, a silicon wafer has been used as a semiconductor substrate for an integrated circuit or a large scale integrated circuit, and one face thereof has been subjected to specular polishing.
FIGS. 1 to 3 of the accompanying drawings show a conventional apparatus employed for such polishing. The apparatus includes a circular rotary surface plate 100, a plurality of mounting heads 102 disposed above the surface plate 100 in circumferentially spaced relation to one another, and a plurality of mounting plates 104 each disposed between a respective one of the mounting heads 102 and the surface plate 100. The surface plate 100 has a polishing pad 100a attached to an upper face thereof. Each mounting head 102 has a circular recess formed in a lower surface thereof, and a shaft 106 is connected to an upper surface of the mounting head 102 at its central portion. Driving means such as a hydraulic cylinder 107 is operably connected to the shaft 106 for causing the mounting heads 102 to move downwardly. Each mounting head 102 is rotatable about and inclinable with respect to the shaft 106. Each mounting plate 104 is made of ceramics, glass material or the like, and has flat opposite surfaces. Each mounting plate 104 is received in the recess of a respective one of the mounting heads 102 with its upper surface being held in contact with an inner planar face of the recess. And, one or more silicon wafers 108 are fixedly secured to the lower surface of each mounting plate 104 with one face of each wafer held in contact with the lower surface of the mounting plate. The silicon wafers 108 may be fixedly secured to the lower surface of the mounting plate 104 by wax applied to the lower surface of the mounting plate 104. Otherwise, the wafers may be secured by means of vacuum adsorption or water adhesion method making use of surface tension.
For polishing the silicon wafers 108 by using such apparatus, a liquid abrasive such as alkaline suspension of a colloidal silica is first poured on the flat upper surface of the surface plate 100. Next, the mounting plates 104 to which the silicon wafers 108 have been fixedly secured are disposed above the surface plate 100 in such a manner that the silicon wafers are interposed between the surface plate 100 and the mounting plates 104, and that that face of each wafer to be polished is brought into contact with the polishing pad of the surface plate 100. Thereafter, the mounting heads 102 as well as the mounting plates 104 are caused to move downwardly by the driving means to urge the silicon wafers 108 onto the upper flat surface of the surface plate 100, following which the surface plage 100 is rotated. Thus, the lower surface of each silicon wafer 108 is polished by the abrasive poured on the upper surface of the surface plate 100.
In the foregoing, the velocity of relative movement of the silicon wafer 108 against the surface plate 100 is greater when the silicon wafer is held in contact with that inner surface portion of the surface plate 100 adjacent to its axis of rotation than when it is held in contact with an outer surface portion thereof. For this reason, among those silicon wafers secured to the same mounting plate, that silicon wafer held in contact with the outer portion of the surface plate 100 is dragged by the rotation of the surface plate 100, so that the mounting plate 104 as well as the mounting head 102 is caused to rotate in the same direction as that in which the surface plate 100 rotates.
The conventional polishing apparatus, however, involves a problem in that it is difficult to polish each silicon wafer so as to have a uniform thickness. Namely, when polishing a silicon wafer by the above apparatus, polishing resistance is exerted between the silicon wafer and the surface plate 100 to cause the mounting plate 104 to be tilted upwardly in the direction of rotation of the surface plate 100 with respect to the shaft 106. A moment defined by a multiplication of the polishing resistance and the height of the lower end of the shaft 106 from the surface plate 100 will be increased if the height of the lower end of the shaft 106 is large, with the result that an inclination of the mounting plate 104 is increased and hence the polished surface of the silicon wafer becomes inclined. Accordingly, the thickness of the wafer thus polished has not been uniform. Further, when the opposite faces of the mounting plate 104 are not parallel to each other, or when the contact surfaces between the mounting head 102 and the mounting plate 104 are undulatory, the urging force transmitted through the mounting head 102 has been exerted unevenly on the entire surface of the mounting plate 104. Consequently, the wafers 108 secured to the mounting plate are caused to be pressed ununiformly against the surface plate 100, resulting in uneven thickness of the wafer.